Method for recovering lost header

ABSTRACT

A method is disclosed that includes sending ( 304 ) a first full header from a source compressor ( 106 ) to a destination decompressor ( 112 ) wherein the source compressor and the destination decompressor are a part of a symmetrical configuration. An IP packet including the full header is compressed into a first compressed header ( 310 ) based on the context established in the first full header. The first compressed header is sent ( 312 ) from the source compressor to the destination decompressor. The compressor receives ( 322 ) a second full header from the destination compressor wherein the second full header indicates that the destination decompressor did not receive the first full header. The second full header can include a modified field or has an incremented generation identification. The source compressor sends ( 336 ) another full header to the destination decompressor.

FIELD OF THE INVENTION

The present invention relates generally to recovering lost headers and,in particular, recovering a full header after an unrecognized compressedheader is received.

BACKGROUND

There are many varieties of header compression including RFC 2507 headercompression. Header compression provides numerous advantages on low- ormedium-speed links. These advantages include improving interactiveresponse times, allowing the use of small packets for bulk data withgood line efficiency, allowing the use of small packets for delaysensitive low data-rate traffic, decreasing header overhead and reducingpacket loss rate over lossy links. Headers that can be compressedinclude TCP, UDP, IPv4 and IPv6 base and extension headers. For non-TCPheaders such as UDP/IP headers, compression slow-start and periodicheader refreshes permit periods of packet discard after loss of a headerthat changes the context. There are hooks for adding header compressionschemes on top of UDP.

Header compression relies on many fields being constant or changingseldomly in consecutive fields belonging to the same packet stream. Thegeneral principle of header compression is to occasionally send a packetwith a full header and subsequent compressed headers refer to thecontext established by the full header and may contain incrementalchanges to the context. An uncompressed header updates or refreshes thecontext for a packet stream. It carries a context identification thatwill be used to identify the context for the packet stream. Full headersfor non-TCP packet streams also carry the generation of the context theyupdate or refresh using a generation identification.

An issue with header compression and header compression using RFC 2507is that the packet containing the full header can be lost between thesource and the destination, which is more likely in wireless mobileenvironments. If the full header is lost, all other packets in thestream will still be compressed by the source but the destination cannotdecompress the compressed stream because it has no information tounderstand the compression.

Recovery mechanisms are known for lost header compression informationwithin RFC 2507 and other header compression techniques. One mechanismis to refresh the context by sending a full header after X packets or Yperiod of time. These mechanisms, however, are not optimal. If X or Yare too large, then many undecodable messages are sent from the sourceand the destination such that time and data is lost. On the other handif X and Y are too small, then there is not much compression becausefull headers are being sent more often.

In view of the foregoing, there is a need to provide an efficientmechanism for refreshing full headers between and a source and adestination to optimize the benefits of header compression.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying figures, where like reference numerals refer toidentical or functionally similar elements throughout the separate viewsand which together with the detailed description below are incorporatedin and form part of the specification, serve to further illustratevarious embodiments and to explain various principles and advantages allin accordance with the present invention.

FIGS. 1A and 1B are an example of a message sequence chart of standardheader compression where the full header sent by the source is receivedby the destination.

FIGS. 2A and 2B are an example of a message sequence chart of standardheader compression where the full header is lost between the source andthe destination and is recovered according to the prior art.

FIGS. 3A, 3B and 3C are an example of a message sequence chart ofstandard header compression where the full header is lost between thesource and the destination and is recovered in accordance with someembodiments of the invention.

FIG. 4 is a flow chart illustrating recovery of a lost full header inaccordance with some embodiments of the invention.

Skilled artisans will appreciate that elements in the figures areillustrated for simplicity and clarity and have not necessarily beendrawn to scale. For example, the dimensions of some of the elements inthe figures may be exaggerated relative to other elements to help toimprove understanding of embodiments of the present invention.

DETAILED DESCRIPTION

Before describing in detail embodiments that are in accordance with thepresent invention, it should be observed that the embodiments resideprimarily in combinations of method steps and apparatus componentsrelated to a method of recovering a full header during headercompression after a full header is lost between a source and adestination. Accordingly, the apparatus components and method steps havebeen represented where appropriate by conventional symbols in thedrawings, showing only those specific details that are pertinent tounderstanding the embodiments of the present invention so as not toobscure the disclosure with details that will be readily apparent tothose of ordinary skill in the art having the benefit of the descriptionherein.

In this document, relational terms such as first and second, top andbottom, and the like may be used solely to distinguish one entity oraction from another entity or action without necessarily requiring orimplying any actual such relationship or order between such entities oractions. The terms “comprises,” “comprising,” or any other variationthereof, are intended to cover a non-exclusive inclusion, such that aprocess, method, article, or apparatus that comprises a list of elementsdoes not include only those elements but may include other elements notexpressly listed or inherent to such process, method, article, orapparatus. An element proceeded by “comprises . . . a” does not, withoutmore constraints, preclude the existence of additional identicalelements in the process, method, article, or apparatus that comprisesthe element.

It will be appreciated that embodiments of the invention describedherein may be comprised of one or more conventional processors andunique stored program instructions that control the one or moreprocessors to implement, in conjunction with certain non-processorcircuits, some, most, or all of the functions of recovering a lost fullheader during header compression described herein. The non-processorcircuits may include, but are not limited to, a radio receiver, a radiotransmitter, signal drivers, clock circuits, power source circuits, anduser input devices. As such, these functions may be interpreted as stepsof a method to perform a method of recovering a full header that hasbeen lost between a source and a destination during header compression.Alternatively, some or all functions could be implemented by a statemachine that has no stored program instructions, or in one or moreapplication specific integrated circuits (ASICs), in which each functionor some combinations of certain of the functions are implemented ascustom logic. Of course, a combination of the two approaches could beused. Thus, methods and means for these functions have been describedherein. Further, it is expected that one of ordinary skill,notwithstanding possibly significant effort and many design choicesmotivated by, for example, available time, current technology, andeconomic considerations, when guided by the concepts and principlesdisclosed herein will be readily capable of generating such softwareinstructions and programs and ICs with minimal experimentation.

The present invention is directed to a method for refreshing a fullheader during header compression when the full header that is sent by asource compressor is lost and not received by a destination compressor.The method includes sending a first full header from a source compressorto a destination decompressor wherein the source compressor and thedestination decompressor are a part of a symmetrical configurationwithin a communication network including a wireless communicationnetwork. The method also includes compressing the full header into afirst compressed header and sending the first compressed header from thesource compressor to the destination decompressor. The sourcedecompressor can receive a second full header from the destinationcompressor wherein the second full header indicates that the destinationdecompressor did not receive the first full header. Upon receipt of thesecond full header by the source, the source compressor sends a thirdfull header from the source compressor to the destination decompressor.In an embodiment of the invention, the first, second and third fullheaders include a context identification and a generation identificationand the generation identification of the second full header isincremented from a previous generation identification to indicate thatthe destination did not receive the first full header, which is neededby the destination to decompress the compressed header for use by thedestination. In another embodiment, the second full header includes amodified generation identification to indicate that the destinationdecompressor did not receive the first full header. The method may alsoinclude detecting that the destination decompressor did not receive thefirst full header upon receiving the second full header by detectingthat a generation identification is incremented. In order for the sourceand the destination to complete header compression, the method caninclude storing the first full header. At the destination, the methodalso includes comparing the second full header to the stored first fullheader to determine if the destination decompressor received the firstfull header. Moreover, comparing the second full header to the storedfirst full header includes determining that the generationidentification of the second full header is incremented from the storedfirst full header.

In another embodiment of the invention, a method is provided thatincludes receiving a decompressed header and determining if thedecompressed header is recognized. If the compressed header is notrecognized, the method includes incrementing a field in a first fullheader and sending the first full header with the incremented field froma destination network to a source decompressor. The method concludes byreceiving a second full header. In an embodiment, the full headersinclude a context identification and a generation identification wherethe incremented field is a generation identification sent from thedestination decompressor to the source decompressor. In an embodiment, adestination includes the destination decompressor and a destinationcompressor and a source includes the source decompressor and a sourcecompressor so that the source and the destination have a symmetricalconfiguration. In another embodiment, the method includes receiving asecond compressed header and determining that the second compressedheader corresponds to the received second full header to ensure that thesecond full header was received. In addition, the method can includereceiving the first full header, storing the first full header andstoring the decompressed header. Moreover, the method may includedetermining if the first compressed header is recognized comprisescomparing the decompressed header to the stored decompressed header.

Turning to FIGS. 1A and 1B, message sequence chart 100 depicts normalheader compression where no packets between a source and a destinationare lost. In an embodiment, FIGS. 1A and 1B illustrate normal RFC 2507header compression procession. In an embodiment, the header compressionis conducted for messages and data sent between a first network 102 anda second network 104. First and second networks 102, 104 include variousnetwork components (not shown) that provide wireless and wiredcommunication between various communication devices (not shown) thatoperate on the network. The communication devices include cellularphones, personal digital assistants, lap top computers, personalcomputers and the like. These components provide communication servicesto multiple users, and there operations are known by those of ordinaryskill in the art. The relevant components within the first and secondnetworks 102, 104 are described more fully below.

First network 102 includes a compressor 106 and a decompressor 108 thatare used to compress and decompress data that is sent and received,including headers, between the first and second networks 102, 104. Inaddition, second network 104 includes a compressor 110 and decompressor112 that are also are used to compress and decompress data that is sentand received, including headers between the second and first networks104, 102. The arrangement of compressors 106, 110 and decompressors 108,112 within the first and second networks 102, 104 provide for asymmetrical configuration of networks between the first and secondnetworks. As the networks are symmetrical, the first network cancompress data and headers which can be decompressed by the secondnetwork, and the second network can compress data and header that can bedecompressed by the first network.

To begin, first network 102 sends 114 an IP packet to the firstnetwork's compressor 106. The IP packet is from the first network 102that is serving as a source network and is to be sent to the secondnetwork 104 that is serving as a destination. As first network 102 andsecond network 104 are symmetrical, second network 104 can serve also asa source network and first network 102 can serve as a destinationnetwork. The IP packet has an address, such as an IP address, for itssource and an address, such as an IP address, for its destination. TheIP packet may also have a source port for its source and a destinationport for its destination. In addition, the IP packet may also include atype-of-service (TOS) data associated with itself.

The compressor 106 stores 116 the full header that is received from thenetwork 102. The full header establishes the context for the packetstream identified by the IP packet. The full header the includes fieldsfor the source address, the destination address, the source port and thedestination port, which can identify the packet stream for the IPpacket. The full header is sent 118 from the source compressor to thedestination decompressor 112 with a context identification (CID) and ageneration identification (generation ID.) The CID is a small uniquenumber identifying the context that should be used to decompress acompressed header. The CID is carried in full headers and compressedheaders so that compressors and decompressors can associate the fullheaders and compressed headers to one another. Context is the state thatthe compressor uses to compress a header and the decompressor uses todecompress a header. The context is the uncompressed version the lastheader sent or received over the link. Generation ID is for each newversion of the context for a given CID that is associated with ageneration. The generation ID is a small number that is incrementedwhenever the context associated with the CID changes.

Upon receipt of the full header, the destination network's 104decompressor 112 stores 120 the full header together with the CID andgeneration ID associated with that header. YH4 After sending the fullheader to the destination network 104, the source network sends 122another IP packet in the packet stream having a similar header as the IPpacket sent in step 114, which includes the same addresses, ports andTOS as before, to the source network's compressor. Based on the contextestablished in step 118, the compressor compresses 124 the IP packetheader into a compressed header. The compressor then sends 126 thecompressed header from the source network 102 to the decompressor 112 ofthe destination network 104 with the CID and generation ID of the fullheader. Upon receipt of the compressed header, the decompressor 112 candecompress 128 the header by associating the CID and generation ID witha stored CID and generation ID. Thus, if the decompressor has a storedfull header with the same CID and generation ID as the compressed headerreceived with that CID and generation ID, the decompressor decompressesthe compressed header by using the stored header. The decompressedheader and IP packet is then sent 130 from the decompressor 112 to thenetwork 104. If no CID and generation ID pairing matches with a storedCID and generation ID then the original full header was lost and notreceived by the destination network 104 and its decompressor 112.

As noted, networks 102 and 104, compressors 106 and 110 anddecompressors 108 and 112 are symmetrical. Thus, steps 114-130 can beconducted by the destination network 104 in steps 132-148. For example,an IP packet is sent 132 from network 104 to its compressor 110 wherethe full header for the IP packet with a CID and generation ID is stored134. The full header and CID and generation ID is sent 136 to thedecompressor 108 of network 102 where the full header is also stored138. An IP packet is sent 140 from network 104 to the compressor 110 anda compressed header for the CID and generation ID is created 142. Thecompressed header with an associated CID and generation ID is sent 144to the decompressor 108 on network 102. The decompressor 108decompresses 146 the compressed header and sends 148 the IP packet tonetwork 102.

FIGS. 2A and 2B are a message control sequence 200 that illustratesheader compression, and header compression for RFC 2507, where a fullheader is recovered by a network 104 and decompressor 112 after beinglost between network 102 and network 104 according to the prior art. Themessage sequence begins by network 102 sending 202 an IP packet to itscompressor 106. As mentioned, the IP packet includes a source address,destination address, source port, destination port and TOS. When a TOSchanges between packets or for other changes in the header, compressor106 determines 204 a new full header is to be sent to decompressor 112and network 104. The full header is assigned a CID and a generation ID.Compressor 106 sends 206 the full header to decompressor 112 and network104. As can be understood, a full header can be lost between thecompressor 106 and decompressor 112. When a full header is lost, thedecompressor cannot decompress a compressed header when it is receivedbecause the decompressor does not have the necessary data such as a CIDand generation ID stored to obtain the full header corresponding thecompressed header.

After the full header is sent, the network 102 sends 208 the compressor106 an IP packet, which is then compressed 209 into a compressed header.The compressed header is sent 210 from the compressor 106 to thedecompressor 112 on network 104. The compressed header is sent with theCID and generation ID that was associated with the header that was lost.At step 212 header decompression fails 212 because the decompressor didnot receive the full header, which is determined when the decompressordoes not recognize the CID and generation ID of the compressed header.As a part of the communication activity between network 104 and network102 and not as a part of an attempt to recover from the lost IP fullheader, network 104 sends 214 an IP packet to its compressor 110. The IPpacket includes a source IP address, a destination IP address, sourceport, destination port. The full header that is assigned a CID andgeneration ID is sent 216 from the compressor 110 to the decompressor108. In an embodiment, a compressed header is also sent 217 from thecompressor 110 to the decompressor 108 where the compressed header hassame the CID and generation ID of the full header. The decompressordecompresses 218 the compressed header upon receipt using the CID andgeneration ID and associated it with the full header having the same CIDand generation ID. The decompressed IP packet is then sent 220 from thedecompressor to the network 102.

In the meantime, network 102 continues to send compressed IP packets tonetwork 104. As the full header was lost, these compressed headerscannot be decompressed by the decompressor 106 of network 104. Headercompression according RFC 2507 includes a refresh option 224. A refreshof the full header occurs after the number of packets using compressedheaders sent since the last full header exceeds a given threshold.Alternatively, a refresh of the full header occurs after the givenperiod of time expires since the last full header was sent. To refreshthe context for the packet stream established in step 204, a full headeris sent 226 using the same CID and generation ID of the full header thatwas lost. If this full header is received by the decompressor 112, thefull header is recovered 228.

FIGS. 3A, 3B and 3C are a message control sequence 300 that illustratesheader compression, and header compression for RFC 2507, where a fullheader is recovered by a network 104 and decompressor 112 after beinglost between network 102 and network 104 according to an embodiment ofthe invention. The message sequence begins by network 102 sending 302 anIP packet to its compressor 106. As mentioned, the IP packet includes asource address, destination address, source port, destination port andTOS. When a TOS changes between packets or for other changes in theheader, compressor 106 determines 304 a new full header is to be sent todecompressor 112 and network 104. The full header is assigned a CID anda generation ID. Compressor 106 sends 306 the full header todecompressor 112 and network 104. As can be understood, a full headercan be lost between the compressor 106 and decompressor 112. When a fullheader is lost, the decompressor cannot decompress a compressed headerwhen it is received because the decompressor does not have the necessaryinformation stored to obtain the full header corresponding thecompressed header.

After the full header is sent 306, the network 102 sends 308 thecompressor 106 an IP packet, which is then compressed 310 into acompressed header. The compressed header is sent 312 from the compressor106 to the decompressor 112 on network 104. The compressed header issent with the CID and generation ID that was associated with the headerthat was lost. Header decompression fails 314 because the decompressordid not receive the full header, which is determined when thedecompressor does not recognize the CID and generation ID of thecompressed header.

In response to failing to recognize the CID and generation ID of thecompressed header, the compressor 110 and decompressor 112 for network104 attempt to recover the full header according to an embodiment of theinvention. By recovering the full header when it is recognized that thefull header was lost, network 104 does not need to wait until the fullheader is refreshed by waiting until the given number of compressedpackets are sent or the allotted time has passed before compressor 106and network 102 refresh the full header. Accordingly, the compressor 110and decompressor 112 increment 315 the generation ID of the next IPpacket that is being sent from network 104 to network 102. The incrementof the generation ID is part of the symmetrical context of the networks102, 104, compressors 106, 110 and decompressors 108, 112.

In view of the foregoing, decompressor 112 increments 315 the generationID for the packet. The packet with the incremented generation ID of thepreviously sent packet from network 104 to network 102 and includes thesource IP address of network 104, destination address of network 102,source port of network 104, destination port of network 102 and theappropriate TOS. This packet is sent 316 from the decompressor 112 tocompressor 110. Compressor 110 finds 318 the appropriate CID, incrementsthe generation ID and can reset the context. Another IP packet is sent320 from network 104 to its compressor 110 with the source IP address,destination IP address, source port and destination port. The compressoruses the appropriate CID and the incremented generation ID and sends 322the full header to the decompressor 108 for network 102.

The decompressor 108 receives the packet with the incremented generationID that is different from the previous generation ID received fromnetwork 104. Based on the incremented generation ID, decompressor 108notifies 324 compressor 106 to reset the context, send a header refreshfor the full header and store a new generation ID. In addition, thepacket is delivered to the compressor. As part of the delivery, thedecompressor 108 sends 326 a packet to the compressor 106 to reset thecontext, which has a source IP address for network 102, a destination IPaddress for network 104, the network's 102 source port and network's 104destination port. In response, the compressor 106 delivers 328 thepacket and the packet is sent 330 from compressor 106 to network 102.Another packet is sent 332 from network 102 to compressor 106 with asource IP address and source port for network 102 and a destination IPaddress and destination port for network 104 as well as the assignedTOS. A full header is sent 334 from network 102 to network 104 becausethe context was reset by the compressor 110 and decompressor 112 ofnetwork 104. The full header is sent 336 with the CID and generation IDso as to recover the context of decompressor 112. Accordingly,decompressor 112 receives a full header because it incrementedgeneration ID when it did not recognize a compressed header and beforethe compressor 106 refreshes the full header during standard operationsof RFC 2507 header compression.

Turning to FIG. 4, a flow chart 400 is shown for full header recoveryaccording to an embodiment of the invention. To begin, the process 400begins with a compressor sending 402 a first full header, which a sourcecompressor has received from a network, to a destination network and adestination decompressor. The header that is sent is sent with adesignated CID and generation ID. In addition to sending the header tothe destination network and decompressor, the source compressor stores404 the header along with the assigned CID and generation ID and otherrelevant data concerning the header and packet. This header, however,can be lost between the source network and the destination network suchthat the destination network does not receive the full header. Aftersending the full header, the source compressor receives another packetto be sent to the destination network and destination decompressor. Thesource compressor compresses 406 the full header into a compressedheader. The compressed header includes the CID and generation header ofthe full header to which the compressed header corresponds. Thecompressed header can also be stored 408 by the source compressor forfuture reference and sends 409 the compressed header to the destinationdecompressor.

At the destination decompressor, the compressed header sent by thesource compressor is received 410. The decompressor determines 412 ifthe compressed header is recognized. In an embodiment, the decompressorcompares 414 the CID and generation ID of the received compressed headerwith CIDs and generation IDs that it has stored from previously receivedfull headers. If the decompressor recognizes the CID and generation ID,the decompressor decompresses 416 the compressed header into a fullheader and sends 418 the full header to the destination network.

If the decompressor does not recognize the CID and generation ID, thedecompressor determines that a full header has been lost and prepares torecover a full header. By actively requesting a full header, thedecompressor does not have to wait until the next full header sent by asource compressor is received by the destination decompressor. To avoidwaiting for the compressor to refresh and send a full header after agiven period of time or after given number of packets with compressedheaders has been sent, the destination network modifies 420 a field in afull header. In an embodiment, the destination network increments 422 afield in a full header that is being sent from the destination networkto the source network. In an embodiment, the destination decompressornotifies 424 the destination compressor to increment the generation IDin the full header. The destination network and compressor sends 426 thefull header with the modified field or the incremented generation ID tothe source network.

The source decompressor receives 428 the full header from thedestination network with the modified field or generation ID and istherefore notified 430 that the full header was not received by thedestination network and destination decompressor. The source compressorthen receives 432 another packet from the network. The source compressorthen sends 434 another full header to the destination network. This fullheader subsequently sent by the source can use the same CID andgeneration ID or it may modify this IDs as required by the networkconditions. After sending this full header, the compressor returns tocompressing 436 full headers into compressed headers and storing 438 thefull headers and compressed headers as required. At the receiver, thedecompressor stores 440 the full header if it is received. When acompressed header is received 442, it compares 444 the CID andgeneration ID of the received compressed header to decompress 446 theheader. If the CID and generation ID are recognized, the full header hasbeen recovered.

In the foregoing specification, specific embodiments of the presentinvention have been described. However, one of ordinary skill in the artappreciates that various modifications and changes can be made withoutdeparting from the scope of the present invention as set forth in theclaims below. Accordingly, the specification and figures are to beregarded in an illustrative rather than a restrictive sense, and allsuch modifications are intended to be included within the scope ofpresent invention. The benefits, advantages, solutions to problems, andany element(s) that may cause any benefit, advantage, or solution tooccur or become more pronounced are not to be construed as a critical,required, or essential features or elements of any or all the claims.The invention is defined solely by the appended claims including anyamendments made during the pendency of this application and allequivalents of those claims as issued.

1. A method comprising: sending a first full header from a sourcecompressor to a destination decompressor wherein the source compressorand the destination decompressor are a part of a symmetricalconfiguration; compressing the full header into a first compressedheader; sending the first compressed header from the source compressorto the destination decompressor; receiving a second full header from thedestination compressor wherein the second full header indicates that thedestination decompressor did not receive the first full header, whereinthe second full header includes a source address, a destination address,a source port, a destination port, a context identification and ageneration identification and further wherein the generationidentification of the second full header indicates that the destinationdecompressor did not receive the first full header; and sending a thirdfull header from the source compressor to the destination decompressorin response to receiving the second full header from the destinationdecompressor.
 2. The method of claim 1 wherein the first and third fullheaders include a context identification and a generationidentification.
 3. The method of claim 2 wherein the generationidentification of the second full header is incremented from a previousgeneration identification.
 4. The method of claim 1 wherein thegeneration identification of the second full header is a modifiedgeneration identification to indicate that the destination decompressordid not receive the first full header.
 5. The method of claim 1 whereinthe headers are UDP headers.
 6. The method of claim 1 further comprisingdetecting the destination decompressor did not receive the first fullheader upon receiving the second full header.
 7. The method of claim 6wherein the detecting the destination decompressor did not receive thefirst full header comprising detecting that the generationidentification of the second full header is incremented.
 8. The methodof claim 1 further comprising storing the first full header.
 9. Themethod of claim 8 further comprising comparing the second full header tothe stored first full header to determine if the destinationdecompressor received the first full header.
 10. The method of claim 9wherein comparing the second full header to the stored first full headercomprising determining that the generation identification of the secondfull header is incremented from the stored first full header.
 11. Amethod comprising: receiving a compressed header; determining if thecompressed header is recognized; incrementing a generationidentification field in a first full header when it is determined thatthe compressed header is not recognized, wherein the first full headerincludes a source address, a destination address, a source port, adestination port, a context identification and the generationidentification; sending the first full header with the incrementedgeneration identification field from a destination decompressor to asource decompressor, wherein the incremented generation identificationindicates that the compressed header is not recognized by thedestination decompressor; and receiving a second full header.
 12. Themethod of claim 11 wherein the second full header includes a contextidentification and a generation identification.
 13. The method of claim11 wherein a destination includes the destination decompressor and adestination compressor and a source includes the source decompressor anda source compressor.
 14. The method of claim 13 wherein the source andthe destination have a symmetrical configuration.
 15. The method ofclaim 11 wherein the headers are UDP headers.
 16. The method of claim 11further comprising receiving a second compressed header.
 17. The methodof claim 16 further comprising determining that the second compressedheader corresponds to the second full header.
 18. The method of claim 11further comprising: receiving the first full header; storing the firstfull header, and storing the decompressed header.
 19. The method ofclaim 18 wherein determining if the first compressed header isrecognized comprises comparing the decompressed header to the storeddecompressed header.